Field of the Invention
The present invention relates to a method for computing MR images of an examination object which executes a cyclic movement and to an MR apparatus for this purpose.
Description of the Prior Art
In the acquisition of MR images of moving organs, the inherent movement of the organ, of the heart for example, must be taken into account and possibly the movement of the organ overall as a result of the movement of the surroundings. This is the case for movement of the heart or of the liver during breathing, wherein this second movement is expectedly a repetitive and almost periodic movement. A first option for the imaging of moving objects is a technique known as the single-shot technique, in which the raw data space (k-space) is read out (filled) completely after the application of an RF pulse and in which the recording of the data is fast enough to freeze the movement. The further recording option is the segmented recording technique, in which the data recording for an image is divided into a number of movement cycles and MR data are recorded only in comparable movement phases. During heart imaging, the breathing and the heart movement must be taken into account. The movement caused by breathing can be minimized by breath-holding techniques or frozen by navigator gating. The first option limits the measurement duration and the second option limits efficiency and increases complexity.
One option for data recording is the so-called CINE data recording for the measurement of the heart muscle movement, in which many MR images for each heart cycle are recorded, and wherein a good constant contrast between heart muscle and blood is needed. This means using a sequence with a good T2/T1 contrast. Another option of image recording is a static tissue characterization, in which the tissue characteristics are determined by measurement of one image per heartbeat. This process usually involves a preparation block in the overall operating sequence, such as a saturation pulse or inversion pulse or a T2 preparation, followed by an optional wait time and the subsequent image data acquisition. The contrast needed for the characterization is created by the preparation block.
Furthermore, the use of contrast media is known, and the tissue characterization can be carried out with or without the use of contrast media. T1 contrasts before and after application of contrast media are of especial significance. An important tissue characterization at the heart is the presentation of scar tissue by a technique known as delayed-enhancement, in which, five to ten minutes after administration of contrast medium, data for a T1-weighted image are recorded such that the healthy myocardium no longer emits any signal, but the scar delivers a bright signal.